1. Field of the Invention
The present invention relates generally to a process for flocculation of clay-containing waste slurries in order to obtain improvement in the settling or filtration characteristics of slurries containing clays. This includes slurries high in acid concentrations (H.sub.2 SO.sub.4, HNO.sub.3 and HCl) as well as slurries containing very little or no acid. More particularly, the invention pertains to a process of selecting a combination of polyethylene oxide with hydroxypropyl cellulose or polyethylene oxide with hydroxyethyl cellulose in order to flocculate clay slurries in aqueous and organic solvents. The invention also entails the use of polyethylene glycol with hydroxypropyl cellulose and polyethylene oxide to flocculate these clay-containing slurries. Flocculation of the clay slurries in an organic solvent at a low pH enables viable recovery of P.sub.2 O.sub.5 values from currently unused sources and allows removal of P.sub.2 O.sub.5 values from leach residues that filter or settle poorly. The invention further entails the use of hydroxypropyl cellulose or hydroxyethyl cellulose with polymers such as polyethylene oxide to increase the settling rates of clay slurries in aqueous or organic solvents. Further still, the invention includes the use of hydroxyethyl cellulose or hydroxypropyl cellulose in combination with a high molecular weight anionic polyacrylamide to flocculate aqueous slurries of phosphatic clay waste and the use of hydroxyethyl cellulose or hydroxypropyl cellulose as settling aids for aqueous phosphatic clay waste slurries.
2. Description of the Prior Art
It is known that the beneficiation of phosphate ore generates a waste stream containing clay and having a solid content of approximately 2 to 6% (Stowasser, W. F. Phosphate Rock. Ch. in Mineral Facts and Problems, 1985 Edition. BuMines B 675, 1985, pp. 579-594; Becker, P. Phosphates and Phosphoric Acid. Marcel Dekker, Inc., 1983, 585 pp; and U.S. Bureau of Mines. The Florida Phosphate Slimes Problem--A Review and a Bibilography. IC 8668, 1975, 41 pp.). This material, also called phosphatic clay waste or phosphate slime, is discarded into waste ponds. Because of the colloidal character of the waste, it settles very slowly and after years it may still contain more than 70% water. Therefore, the manner of disposal, which ties up large quantities of land and water, is an important environmental concern. Because of this, finding ways to rapidly dewater clay wastes has been the subject of much research.
In this regard, polymers such as polyethylene oxide (PEO) and anionic polyacrylamides have shown promise (Scheiner, B. J. and G. M. Wilemon. Applied Flocculation Efficiency: A Comparison of Polyethylene Oxide and Polyacrylamides. Paper in Flocculation in Biotechnology and Separation Systems, ed. by Y. A. Attia (Inter. Symp. on Flocculation in Biotechnology and Separation Systems, San Francisco, CA, Jul. 28-Aug. 2, 1986). Elsevier, 1987, pp. 175-185.).
Another problem is that a considerable amount of the phosphate present in untreated ore is lost when the waste is discarded. In the case of waste ponds in Florida, said ponds approximately cover 90,000 acres and contain about 1/3 of all of the phosphate ever mined. This represents millions of tons of unrecovered phosphate.
Because of the fact that current phosphate deposits are decreasing in quality, the ability to recover phosphate values from the waste would greatly increase the domestic reserve.
Some efforts have been made to circumvent the loss of this phosphate by directly acidulating phosphate ore using H.sub.2 SO.sub.4 (White, J. C., T. N. Goff, and P. C. Good. Continuous-Circuit Preparation of Phosphoric Acid From Florida Phosphate Matrix. BuMines RI 8326, 1978, 22 pp; White, J. C., A. J. Fergus, and T. N. Goff. Phosphoric Acid by Direct Sulfuric Acid Digestion of Florida Land-Pebble Matrix. BuMines RI 8086, 1975, 12 pp; Good, P. C., T. N. Goff, and J. C. White. Acidulation of Florida Phosphate Matrix in a Single-Tank Reactor. BuMines RI 8339, 1979, 16 pp.).
While this acidulating technique with H.sub.2 SO.sub.4 does leach 90+% of the phosphate from the ore, there are two distinct disadvantages to this approach. First, the crude acids prepared by direct acidulation are not of sufficient purity to be considered fertilizer precursors (the major end use of wet-processed acid); and secondly, the action of the H.sub.2 SO.sub.4 on the matrix generates an insoluble residue having a very small average particle size, and this makes solid/liquid separation of the acid from the residue slow and difficult.
Recent efforts at the BUREAU OF MINES involve leaching phosphate values from wastes using H.sub.2 SO.sub.4 in the presence of methanol. While the organic solvent serves to depress the solubility of impurities and thereby improve the quality of the crude acid generated, the leach slurries still contain an insoluble clay component that must be separated from the crude acid product. In this process, leach slurries may be filtered, but the filtrations are slow and the filter cakes are tight and very difficult to wash, and this results in a loss of phosphate as well as handling problems.